Major depressive disorder (MDD) is a debilitating disorder of altered mood regulation that is precipitated by chronic stress, modulated by serotonin and of unknown molecular pathophysiology. Four prominent risk factors have been consistently reported to influence rates of vulnerability to develop depression: sex, genetic make-up, prior MDD episodes, and early life stress; however no single animal model has comprehensively combined these vulnerability factors. Using the unpredictable chronic mild stress (UCMS) protocol and behavioral assessments of anxiety/depression-like behaviors in serotonin transporter (SERT) mutant and control mice, I have modeled the interactions of sex, SERT genetic make-up, and stress in eliciting depressive-like behaviors, including the increased vulnerability to clinical depression observed in human female subjects and the risk that is conferred by low SERT levels. Furthermore, gene microarray studies performed by our laboratory on the amygdala of postmortem human MDD subjects and UCMS-treated mice have identified ~40 genes whose changes are specific to human MDD and rodent UCMS, and reversed by antidepressant treatment in rodents, thus representing a critical pool of genes differentially expressed according to altered mood. Therefore, in addition to modeling aspects of human susceptibility to develop depression (sex and SERT genetic make-up), the rodent UCMS paradigm induces molecular changes that are predictive of ?depressive states? across species. Using a comprehensive UCMS-based experimental design in the mouse, I will first confirm the role of sex and SERT as risk factors to UCMS-induced altered mood regulation and then extend my investigation to two additional factors: disease recurrence and early life stress (Aim 1). Using samples generated in Aim 1, I will then begin characterizing neurobiological phenotypes that underlie the UCMS-evoked depressive-like state (Aim 2), including neuroendocrine changes and quantitative assessment of the amygdala gene expression signature. I hypothesize that while risk factors may differentially increase the susceptibility for developing MDD, the ultimate state of altered mood regulation affects a common set of biological and/or molecular disturbances. Results from these studies will begin characterizing changes in these putative common molecular and neuroendocrine disturbances, will reveal the relative contribution of these susceptibility factors to this common phenotype, and will form the basis of future causative studies aimed at investigating the cellular and molecular basis of MDD.